1. Field of the Invention
This invention relates to electronic detectors for detecting faults and, more particularly, to circuit breakers for detecting or locating arc faults in an ac power system. The invention also relates to methods for locating arc faults in an ac power system.
2. Background Information
An aircraft electrical system typically consists of multiple 115 VAC 400 Hz three-phase voltage sources (e.g., one generator per engine, an auxiliary turbine power generator and a ground power external connection). The voltage sources or power supplies are all fed through main circuit cables to a power distribution center located behind the cockpit. At the power distribution center, main power is switched by a transfer circuit to a number of feeder circuits, in order that any feeder circuit may be fed from any voltage source. This allows for power continuity should an engine fail. Typically, two feeder circuits are employed, one for the left side and one for the right side of the aircraft.
Each feeder circuit typically has many individual branch circuits, each of which is associated with a corresponding load. A typical small aircraft has about 200 branch circuits, while larger aircraft may have about 400 to 600 branch circuits. Each branch circuit feeds a dedicated load through a single wire protected by a circuit breaker located in the power distribution center. The return current path is through the aircraft's metal frame rather than a separate neutral wire as is used in commercial and residential wiring. The individual branch circuit wires are grouped into cable bundles with typically more than 100 wires in a bundle. The bundles start at the power distribution center and are routed from front to back through the aircraft, often along the aircraft's outer skin between the skin and the insulation.
These wire or cable bundles are installed during the aircraft's construction. The locations of the wires in an aircraft make them very hard to see after the aircraft is built; hence, a wire fault is hard to locate.
Ac power generation and distribution systems for aerospace, industrial and residential applications have historically been designed and optimized to operate with voltage sources of a fixed amplitude and frequency. Examples of this include the 60 Hz system used for power generation and distribution in the United States, a similar 50 Hz system in Europe, and the 400 Hz power distribution systems which are standard on most aircraft. There is interest in developing variable frequency power generation and distribution systems for aircraft, with all electrical loads on board the aircraft being designed to operate over a range of ac source voltage frequencies (e.g., from about 200 Hz to about 800 Hz). See, for example, U.S. patent application Ser. No. 10/135,591.
A known technique for arc fault protection in 60 Hz ac power systems detects the random step changes in ac current caused by the striking of an arc. When a time attenuated accumulation of the step changes reaches a selected value, an arc signal is generated. Aircraft electrical systems utilize small gauge wires which can be very long and, therefore, have substantial resistance. As a result, a load close to the generator can draw a normal current that far exceeds a fault current on the remote part of the wiring. Thus, a fault current may draw a couple of thousand amps if close to the generator but only perhaps 30 or 40 amps if in a remote section of the wiring.
U.S. patent application Ser. No. 09/621,253 describes an arc fault detector suitable for aircraft ac electrical systems or other ac systems operating at higher frequencies, such as 400 Hz. The arc fault detector generates a cumulative sum of amounts by which the ac current in each most recent cyclic interval exceeds the current in the immediately preceding half cycle in absolute magnitude. An arc fault indication is generated when a time attenuated value of this cumulative sum reaches a selected level.
Wire faults arise from the occurrence of a wire insulation failure, which allows normally isolated conductors at different potentials to touch resulting in an arc fault. Two such conditions are shown by a 115 VAC wire-to-frame fault 1 between wire WA and airframe AF in FIG. 1, and a 200 VAC wire-to-wire (phase-to-phase) fault 2 between wires WA and WB in FIG. 2. No faults exist in wires WB and WC of FIG. 1 and in wire WC of FIG. 2.
The current waveshape of an arc fault is shown in FIG. 3. This shows a typical current waveform, which is observed when a carbon-steel blade cuts through a 16 AWG SPT-2 cord, with the available current being 100 A. Unlike overload and overcurrent faults, the current of an arc fault is sporadic. The conduction between conductors is through an electrical arc that tends to be unstable. The peak current is determined by the instantaneous line voltage of the voltage source minus the arc voltage divided by the wire impedance from the arc fault back to the voltage source.
When an arc fault occurs, it is very difficult to find because the wire bundles are typically inaccessible. Even if the wire bundle is viewable, the faulted wire itself may not be visible, since it may be inside the bundle.
A simple and reliable apparatus and method for locating a wiring fault is needed, in order that the fault can be located and repaired. For aircraft applications, this permits the aircraft to be returned to service as soon as possible.